A. Field of the Invention
This invention relates generally to a keyboard device for a musical instrument, and specifically to a keyboard switching device for an electronic musical instrument.
B. Background Art
A keyboard type electronic musical instrument is generally constructed to open or close a tone signal source by key depressing pressure. Moreover, a touch-responsive effect is generally added to control the amplitude of the tone signal in response to key depressing speed. Such touch-responsive control depends on measuring the time difference between the start and end of key depression. This time measurement may be done in several ways. For example, in may be done by way of two switches--one normally closed (NC) and the other normally open (NO)--which are connected to a charging/discharging circuit. Immediately upon key depression the NC switch is opened, causing a charged capacitor in the circuit to begin discharging. At the end of key travel the NO switch is closed, causing the remaining voltage on the capacitor to be sampled. The faster the key depression, the larger the remaining voltage on the capacitor and the louder the corresponding note will sound. Thus, there is frequently a need for NC and NO switches in an electronic musical instrument.
In prior art electronic musical instruments, switches have been formed of a stationary contact piece arranged on a base plate, and a movable contact piece arranged above the stationary piece. The movable piece has been formed of a resilient material, such as rubber, and a conductive portion, such as conductive rubber, carbon, silver, etc. adhered thereto. In the normally open state, the conductive portion was maintained at the upper position by the tension inherent in the resilient material with the result that it was separated in space from the stationary piece. Over extended usage, the resilience of the material would decrease or tend to set causing the conductive portion to come in contact with the stationary piece. Once setting occurred, the NO switch had to be replaced. This, of course, increased the cost of repair and maintenance of the instrument.
Due to the setting tendency of the material, it has not been feasible to construct a NC flexible switch. NC flexible switches made in the standard way would tend to set rather quickly, since with a key in its normal rest position the NC switch would be compressed closed most of the time. As a result of the constant pressure, upon depression of the key, the NC switch tended to remain closed.
Thus, it has not been possible to construct a NC switch from flexible or resilient material. It has also not been possible to construct a NO switch from flexible or resilient material which has long endurance characteristics and does not set after prolonged use.